Cut the pork belly into ~1.25-inch cubes. Coat with Japanese BBQ sauce as a binder, then season all sides with SPG rub.
Smoke at 275°F until they develop a nice bark and reach ~180°F internal (about 2–2.5 hours).
For the sauce, add all ingredients to a pan over medium-low heat. Mix well and let simmer, stirring frequently, until thickened (~3–5 minutes). Set aside.
Remove the pork belly from the smoker and place in a pan. Top with the sweet & sticky sauce, mix well, then cover tightly with foil.
Place back on the smoker until probe tender (~203–205°F internal / 1–1.5 hours). Remove the foil and let them cook another 20–30 minutes to let the sauce tack up.
Remove from the smoker, let cool a few minutes, then top with bang bang sauce, sesame seeds and green onions and serve.
Measurements are estimated; adjust to your preference.